IRSSI support for Off-The-Record (http://www.cypherpunks.ca/otr/)
messaging, allowing you to have private conversations over instant
messaging by providing:

Encryption
 * No one else can read your instant messages.

Authentication
 * You are assured the correspondent is who you think it is.

Deniability
 * The messages you send do not have digital signatures that are
   checkable by a third party. Anyone can forge messages after a
   conversation to make them look like they came from you. However,
   during a conversation, your correspondent is assured the messages
   he sees are authentic and unmodified.

Perfect forward secrecy
 * If you lose control of your private keys, no previous conversation is
   compromised.
